This invention relates, in general, to semiconductor devices, and more particularly, to a metallization process for semiconductor devices.
Metal layers must reliably cover via having very high aspect ratios (the ratio of the depth to the width) to permit the design of high density integrated circuits. Steps having very great depths are also a problem. Metal step coverage is not a great problem when trying to cover less aggressive geometries, vias having low aspect ratios or steps having small depths. Adequate step coverage can be obtained on less aggressive geometries when the metal layer is deposited at a "cold" temperature of approximately less than or equal to 200.degree. C. However, as step heights, via depths and aspect ratios have increased on integrated circuits, metal which is deposited at a cold temperature does not produce good step coverage due in part to severe cusping at the corners of vias, which forms because the deposition is not conformal, and also due to physical shadowing from the via structure itself. The cusping causes the via to be closed before the metal has actually filled the via. One solution to this problem is to increase the temperature at which the metal layer is deposited to provide increased metal mobility to achieve acceptable step coverage. This high temperature is typically in the range of 400.degree. C. to 500.degree. C. However, as the temperature at which metal is deposited is increased, another problem has arisen. The use of higher deposition temperatures causes unpredictable voiding or discontinuities in the metal layer. Voiding occurs when there is insufficient metal in a certain area to support continuous grain growth. This problem is worsened when the aspect ratios of vias increase, because the amount of metal deposited in the bottom corners and on the sidewalls decreases due to shadowing and cusping at corners of the via. Although an increase in the deposition temperature was supposed to improve metal atom surface mobility to fill deeper vias, the increase in temperature frequently causes the early formation of widely spaced grains that lead to the formation of voids. Insufficient filling of a via caused by cusping or voiding of the metal layer is undesirable because it may cause devices to be inoperable, or at the minimum, to have reliability problems.
Adequate step coverage has been achieved by the use of flared or tiered vias. Flared or tiered vias reduce the cusping at the corners of a step or via, however valuable semiconductor area is consumed. Using low metal deposition rates also can help to achieve adequate step coverage, however, throughput is severely lowered, which makes the process too costly.
A multi-step deposition process has been used to improve step coverage in vias or steps having increased depths or aspect ratios. This process entails the deposition of a thin portion of the metal layer at a cold temperature. After the thin portion of the metal layer has been deposited, the temperature is ramped up to approximately 400.degree. C. to 500.degree. C., where the bulk of the metal layer is deposited. In this process it is thought to be desirable to deposit only a thin portion of the metal layer as early as possible so that the bulk of the metal layer can be deposited at the higher temperature under high mobility conditions in order to reduce voiding. The thin portion of the metal layer deposited at a cold temperature is needed to provide for surface diffusion. This process, however, has been found to still produce a large fraction of voiding by not supporting continuous grain growth.
Another attempt at providing adequate step coverage entails depositing all the metal layer, and then planarizing the metal layer by melting or reflowing the metal with a laser. This process, however, is not very reproducible with aluminum or aluminum alloys because a native oxide forms on the aluminum, thus preventing planarization. In addition, aluminum requires a high optical pulse energy due to its high reflectance, and variations in surface topography can increase absorbed power, thus causing damage.
The above described processes used conventional means of depositing metal films with the use of an evaporator or a sputterer. A metallization process using an unconventional deposition technique was disclosed in a paper by T. M. Lu, P. Bai and A. S. Yapsir, entitled "Partially Ionized Beam Processing: Via Filling and Planarization," published in Semiconductor Research Corporation TECHON '88, Oct. 12-14, 1988, Dallas, Tex., extended abstracts pp. 75-78. The authors found that applying a partially ionized beam (PIB) deposition technique produced metal layers having adequate step coverage and planarization. The PIB process utilized a two-step deposition with different substrate temperatures. Part of the metal layer is deposited at a temperature of 150.degree. C., and after the via or trench is filled, the remaining metal layer is deposited at a temperature of 300.degree. C. During the first step, the filling of the via is completely non-conformal; the via is filled without much cusping unlike with metal deposited by conventional means at a cold temperature. A higher temperature is used to achieve planarization of the metal layer. If no ions are applied during the deposition of metal at 300.degree. C., a very rough film would form. This process does produce adequate results, however, is not very manufacturable because the deposition rate, 13 angstroms/sec, is very low, and a substrate bias of several kV is needed to achieve via filling and planarization. A low deposition rate is undesirable because of low throughput and risk of gaseous inclusion into the metal layer, which makes the metal more porous and less reliable. A very high substrate bias is also undesirable because the process is hard to maintain at a constant voltage and high voltages may damage semiconductor devices.
By now it should be appreciated that it would be advantageous to provide an improved metallization process using conventional deposition means to achieve adequate metal step coverage.
Accordingly, it is an object of the present invention to provide an improved method of depositing a metal layer which provides a high degree of step coverage over high steps and into vias having very high aspect ratios.
Another object of the present invention is to provide an improved method of depositing a metal layer without the formation of voids.
A further object of the present invention is to provide an improved method of providing adequate metal step coverage utilizing the metal's recrystallization, bulk diffusion and grain growth properties to reduce the formation of voids.
Yet another object of the present invention is to provide a metallization process that has high throughput.